1. Field of the Invention
The present invention relates to an image drawing apparatus and an image drawing method. More specifically, the present invention relates to an image drawing apparatus and an image drawing method for forming a two-dimensional pattern represented by image data on an image drawing surface by using a plurality of image drawing heads.
2. Description of the Related Art
There have been known various kinds of image drawing apparatuses each comprising an image drawing head for forming a desired two-dimensional pattern represented by image data on an image drawing surface. As a representative example of such an image drawing apparatus, an exposure apparatus for forming a desired two-dimensional pattern on an exposure surface of a photosensitive material or the like by using an exposure head in order to generate a semi-conductor substrate or printing plate can be listed. Such an exposure head in an exposure apparatus generally has a pixel array comprising a plurality of pixels for generating light spots forming a desired two-dimensional pattern, such as a light-source array or spatial light modulators. The desired two-dimensional pattern can be formed on an exposure surface by causing the exposure head to operate with movement relative to the surface.
In such an exposure apparatus, covering a sufficiently large exposure surface with a single exposure head is in some cases difficult for a configuration of a pixel array or the like, as in the case where a digital micro-mirror device (DMD) of a generally available size is used as spatial light modulators, for example. Therefore, an exposure apparatus using a plurality of exposure heads in parallel has been proposed. Furthermore, in order to improve resolution in the direction that is perpendicular to the direction opposite to the direction of exposure surface movement (hereinafter, the direction opposite to that of exposure surface movement is referred to as the scanning direction), an exposure apparatus using exposure heads has been proposed wherein pixel arrays having two-dimensionally arranged pixels are installed diagonally to the scanning direction.
For example, Japanese Unexamined Patent Publication No. 2004-009595 describes an exposure apparatus that includes exposure heads laid out diagonally to the scanning direction and comprises DMD's having micro-mirrors laid out in a rectangular grid shape. The exposure heads are installed in the exposure apparatus in such a manner that triangular areas formed as a result of diagonal installation of the DMD's if both ends of each of the DMD's are cut by lines parallel to the direction of scanning are complemented each other by the neighboring DMD's placed along the direction perpendicular to the scanning direction.
Furthermore, Japanese Unexamined Patent Publication No. 2003-195512 describes an exposure apparatus whose exposure areas formed by DMD's are shaped into parallelograms. In this exposure apparatus, exposure heads comprising the DMD's formed in a rectangular grid shape are placed perpendicular to or slightly diagonal to the scanning direction, and the exposure areas of the neighboring DMD's laid out in the direction perpendicular to the scanning direction overlap with a predetermined width. In areas corresponding to the overlaps of the exposure areas, the number of micro-mirrors to be driven in the DMD's is gradually increased or decreased in a predetermined proportion. In this manner, the exposure areas formed into parallelograms can be realized.
However, in the case where exposure is carried out by using exposure heads each comprising a pixel array having two-dimensionally placed pixels whose columns are placed diagonally to the scanning direction, fine adjustment of positions and angles of the heads relative to each other is generally difficult, and slight deviation from ideal positions and angles is often observed. Furthermore, distortion may be observed in a pattern actually formed on an exposure surface, due to various kinds of optical aberration between the pixel arrays and the exposure surface and distortion of the pixel arrays itself. Therefore, even in the case where an exposure apparatus is used in such a state that exposure areas formed by pixel arrays are smoothly relayed in an ideal state with no such distortion and deviation, complete elimination of such distortion and deviation is extremely difficult in practice. Consequently, uneven resolution and density do occur in areas drawn by neighboring heads (hereinafter referred to as inter-head relay areas) in the two-dimensional pattern on the exposure surface.
In order to solve this problem, improvements may be made in accuracy of fine adjustment of positions and angles between exposure heads and in adjustment of optical system. However, pursuing this accuracy swells manufacturing cost.
The same problem can also occur in an image drawing apparatus of other types, such as an inkjet printer comprising an inkjet recording head used for drawing an image by propelling droplets of ink to a surface whereon the image is drawn.